Second Half of Biology 1108October 15, 2013Chapter 22: EvolutionA species consists of a range of variantsNatural selection depends on the differential success- in terms of surviving and reproducing- of variantsBefore Darwin, variation was irrelevant but after it was essential to the evolutionary processScientific theory is built using inductive logic. It is improvable but falsifiable and describes a lot with just a little. 21.1 Genetic VariationPhenotype: an observable traitDependable upon:1.) A genotype: the set of alleles possessed by the individual at relevant genetic loci2.) The environment in which the individual livesSpecies: individuals that can exchange genetic material through interbreeding. Theycan share alleles with one another by reproductionGene pool: all of the alleles present in all individuals in a speciesPopulation genetics is the study of the genetic variation in natural populations, which are interbreeding groups of organisms of the same species living in the same species living in the same geographical areaTwo sources:1.) Mutationa. Somatic: occurring in the body’s tissuesb. Germ-line: occurring in the reproductive cells and therefore passed tothe next generation. Appears in every cell of an individual derived from the fertilization involving mutation-bearing gamete, and thus in its descendentsThese mutations can be:-Deleterious-Neutral-Advantageous2.) Recombination21.2 Measuring Genetic VariationAllele Frequencies: The rates of occurrence of alleles in populationsThe allele frequency of an allele x is simply the number of x’s present in the population divided by the total number of allelesFixed: when a population exhibits only one allele at a particular geneThus, the allele frequencies of A and a provide a measure of genetic variation at one gene in a given populationWays to measure genotype and allele frequencies in populations:-Observable traits: Doesn’t always work alone because there are many genes that code for one genotype and the phenotype also relies on the environment, not just the genotype alone.-Gel electrophoresis-DNA sequencing: only with this did we have an unambiguous way of detecting all genetic variation in a stretch of DNA, whether is coding region or not21.3 Evolution and the Hardy-Weinberg EquilibriumEvolution is the change in allele or genotype frequency over time and the change in genetic makeup of a population over time.Populations evolve, not individualsHardy-Weinberg Equilibrium: the situation in which evolution does not occur. Specifies the relationship between allele frequencies and genotype frequencies when a number of key conditions are met, and therefore, evolution is not occurring. These are the primary causes of evolution:- There can be no differences in the survival and reproductive success of individuals (no natural selection)- Populations must not be added to or subtract from by migration- There can be no mutation which is the ultimate source of genetic variation, so it is a key input on which other evolutionary mechanisms act- The populations must be sufficiently large to prevent sampling errors (a change in the frequency of an allele due to the random effects of a small population size is genetic drift)- Individuals must mate at randomThese are represented by the genotypes and the frequencies but only if the conditions are metNot only does this provide a means of converting between allele and genotype frequencies, but it also serves as an indicator that something interesting is happening in a population when it is not upheld21.4 Natural Selection- Results in allele frequencies changing from generation to generation according to the allele’s impact on the survival and reproduction of individuals. - New mutations that are deleterious and eliminated by natural selection have no long-term evolutionary impact; ones that are beneficial can result in adaptation to the environment over timeFitness: a measure of the extent to which the individual’s genotype is represented in the next generationPositive Selection: natural selection that increases the frequency of a favorable alleleNegative Selection: natural selection that decreases the frequency of a harmful alleleBalancing Selection: natural selection that acts to maintain two or more alleles in a population. This can be in different areas, where one allele is favored in a dry area and the other allele is favored in wet areas.- Another form, heterozygote advantage, is when the heterozygote’s fitness is higher than that of either of the homozygotes, resulting in selection that ensures that both of the alleles remain in the population at intermediate frequencies. This is exemplified with Africa and MalariaThree patterns:1.) Stabilizing: maintains the status quo and acts against extremes. Ex: Birth weight2.) Directional: leads to a change in a trait over time by increasing the frequencyof one allele (eventually becomes fixed). Ex: Bacterial Drug Resistance-Artificial: analogous to natural selection but the competitive factor is removed to choose attractive traits3.) Disruptive: operates in the favor of extremes and against intermediate forms and can lead to the creation of a new speciesNonrandom mating impacts the genotype frequency through the phenotype- This is when indiv are inclined or unable to mate indiscriminately- Common in nature- Two mechanisms:o Interbreeding: mating between relatives that increases the frequency of homozygotes and reduces the freq of heterozygotes in each generation. This causes interbreeding depression which is thedecline in fitness that takes place when homozygosity increases and heterozygosity decreaseso Sexual Selection: promotes the traits that increase an individual’s access to reproductive opportunities that can act against natural selection Females may be choosier in their mates because they invest more in their offspring than males do. So they choose on the basis of physical characteristics that signal male genetic quality Sexual dimorphism refers to any trait that differs between males and females of the same species (peacocks)21.5 Migration, Mutation, & Genetic DriftThese are the forces that can cause allele frequency to change and they do not lead to adaptations (non-adaptive mechanisms)Migration: the movement of individuals from one population to another resulting in gene flow, the movement of alleles from one population to another- The consequence of this is the homogenizing of